Double-fed asynchronous machines in the power range from 20 MVA to 500 MVA can be used for variable-speed energy production. These machines are distinguished by a distributed three-phase winding on the rotor. The rotor winding consists of individual bars which are embedded in slots in the rotor laminations. The individual bars are connected in the winding head to form a winding. The arrangement of the bar connections is uniformly distributed around the circumference. As a result of the rotation of the rotor, the winding heads are subjected to centrifugal forces, against which they have to be mechanically secured by means of winding head retention systems. In principle, there are three types of winding head retention systems:
1. Fixing by means of a steel cap, as is the case with turbogenerators.
2. Fixing wherein a steel cable, wire or plastic film is wrapped around the whole winding head.
3. Fixing by means of bolts, screws or U-shaped brackets.
Such an asynchronous machine 10 is reproduced in section in highly simplified form in FIG. 1. It comprises a rotor 19 which can be rotated about an axis 18 and is encompassed concentrically by a stator lamination stack 14 with corresponding stator winding and a stator winding head 17. The rotor 19 comprises a central body 11 which merges with a shaft 11′ at each end. The central body 11 is surrounded by a rotor lamination stack 12 in which the rotor winding 13 runs. Slip rings 15, which are used to supply the rotor winding 13 with current, are arranged on one of the shafts 11′. The rotor winding head 16, which is shown once more enlarged in vertical orientation in FIG. 2(a), is equipped with a winding head retention system 29 which, according to FIG. 2(a), is fitted with retention elements which run radially through the rotor winding head 16 in the form of threaded rods 20 or threaded bolts or similar, and which are fitted with nuts 21.
The retention elements 20, 21 absorb the centrifugal forces acting on the rotor winding head 16 during the operation of the machine and pass them into the solid part of the rotor arranged below the winding head. It can now come about that the heavily mechanically loaded retention elements 20, 21 break or lose their mechanical integrity in some other manner during operation over a longer period. However, it may also be that the nuts 21 or screw fittings come loose and in this way the function of the retention elements 20, 21 is lost. If, as part of this failure, some of the retention elements 20, 21 come loose and are flung outwards, the machine can incur consequential damage which leads to extended downtime of the machine and therefore to high consequential costs.
It is desirable to either prevent such a failure of the retention elements 20, 21 altogether or at least keep the consequential damage within limits. Furthermore, it would be of advantage if abnormal changes in the winding head could be detected in good time and the machine could be switched off at an early stage in order to limit damage or to prevent it occurring in the first place.